With the emergence of 3G mobile telephony, new packet-based communication technologies have been developed to support multimedia communication. For example, GPRS (General Packet Radio Service) and WCDMA (Wideband Code Division Multiple Access) technologies support wireless multimedia telephony services involving packet-switched communication of data representing images, text, documents, animations, audio files, video files, etc., in addition to traditional circuit-switched voice calls.
Multimedia services typically entail transmission of encoded data representing text, documents, images, audio files and video files in different formats and combinations. The term “multimedia” will be used in this description as generally referring to any choice of media communicated by using the packet based IP (Internet Protocol) transport technology.
A network architecture called “IP Multimedia Subsystem” (IMS) has been developed by the 3rd Generation Partnership Project (3GPP) as an open standard for handling multimedia services and sessions in the packet domain. IMS is a platform for enabling services based on IP transport, more or less independent of the access technology used, and is neither restricted to any specific services. Thus, an IMS network controls multimedia sessions but is not used for the actual transfer of payload data which is routed over access networks and any intermediate transport networks.
FIG. 1 is a simplified schematic illustration of a basic network structure for providing multimedia services by means of an IMS service network. A first mobile terminal A is connected to a first radio access network 100 and communicates with a second mobile terminal B connected to a second radio access network 102, in a communication session S involving one or more multimedia services. There may also be an intermediate backbone network, not shown, as well linking the access networks 100 and 102.
An IMS network 104 is connected to the first radio access network 100 and handles the session with respect to terminal A. In this figure, a corresponding IMS network 106 handles the session on behalf of terminal B, and the two IMS networks 104 and 106 may be controlled by different operators. Alternatively, terminals A and B may of course be connected to the same access network and/or may belong to the same IMS network. Terminal A may also communicate with a fixed terminal or computer or server instead, e.g. for downloading some media over the Internet, as long as the other party is capable of SIP communication. Moreover, if a terminal is roaming in a visited access network, multimedia services are handled by the terminal's “home” IMS network.
The session S shown in FIG. 1 is managed by specific nodes in each IMS network, here generally referred to as “session managing nodes” 108. These nodes typically include S-CSCF (Serving Call Session Control Function), I-CSCF (Interrogating Call Session Control Function) and P-CSCF (Proxy Call Session Control Function). Each IMS network 104,106 also includes one or more application servers 110 for enabling various multimedia services. Further, a main database element HSS (Home Subscriber Server) 112 stores subscriber and authentication data as well as service information, among other things. IMS network 106 is basically similar to network 104. The various specific functions of the shown network elements 108-112 are generally known in the art, but are not necessary to describe here further to understand the context of the present invention. Of course, the IMS networks 104,106 contain numerous other nodes and functions not shown here for the sake of simplicity.
A specification for handling sessions in IMS networks has been defined called “SIP” (Session Initiation Protocol, according to the standard IETF RFC 3261). SIP is an application-layer control protocol for signalling, to create and generally handle sessions over a packet-switched logic. The SIP standard is thus used by IMS systems and SIP-enabled terminals to establish and control IF multimedia communications. SIP itself does not provide multimedia services, but rather makes available a set of primitives that other protocols or applications can use to actually implement such services.
For example, a message called “INVITE” is defined in SIP to initiate a multimedia session during session set-up, when a certain application has been invoked. The SIP INVITE message typically includes, among other things, a description of the session, i.e. information on required codec(s) and other communication parameters needed for the forthcoming session.
SIP uses an additional protocol called Session Description Protocol, SDP, for describing multimedia sessions, which can be embedded as a self-contained body within SIP messages. SDP can be used by terminals to exchange information regarding their specific capabilities and preferences, in order to negotiate and agree on which session parameters, codec's in particular, to use during a forthcoming multimedia session, as is well-known in the art. Preferred or required session parameters may be indicated as attributes referred to as “preconditions” in the SDP information.
Many mobile applications require a certain Quality of Service QoS in order to provide a satisfying result to end-users. For UMTS networks, four main traffic classes have been defined: “conversational class”, “streaming class”, “interactive class” and “background class”, in order to classify different needs regarding bit rates and delays. These traffic classes are primarily distinguished by their requirements regarding transfer delays, such that applications of the conversational class tolerate only small delays, sometimes also referred to as “real-time”, whereas the background class is applied to the least delay-sensitive applications, sometimes also referred to as “best effort”.
The selection of a UMTS traffic class for an application is used for assigning a suitable physical channel in the access network, generally referred to as a RAB (Radio Access Bearer), in order to optimise the scarce radio recourses in the access network, whilst maintaining acceptable quality for the end-user.
Mobile terminals capable of multimedia are typically configured to identify for each inherent application, a UMTS traffic class, as schematically illustrated in FIG. 2. Thus, a mobile terminal may hold a number of applications 200, denoted as A1, A2, A3, A4, A5 . . . . A mapping function 202 in the terminal translates each application to a certain UMTS traffic class 204, of which only two are shown here. In this case, applications A1, A2 and A4 are mapped to the same UMTS traffic class 2, since they have similar requirements regarding bit rate and delay, whereas applications A3 and A5 are mapped to UMTS traffic class 1. In this way, several applications with similar characteristics may be mapped onto the same RAB, fulfilling their requirements.
However, before a mobile terminal can exchange any SIP messages with the IMS network, a “PDP (Packet Data Protocol) context” must be established for the terminal. Basically, a PDP context can be activated once the terminal has been powered on. Activating a PDP context for a mobile terminal includes allocating a temporary IP address to the terminal, to be able to communicate data packets with the terminal. A PDP context also means that a physical channel is allocated in the access network, generally referred to as a RAB (Radio Access Bearer), for IP communication. Thus, SIP messages can only be sent over a PDP context.
FIG. 3 illustrates the gradual activation of a mobile terminal A about to communicate multimedia with another party B, involving basically five stages 3:1-3:5 as illustrated, each comprising various messages back and forth. These messages are well-known in the art and will not be described in any detail. Terminal A is located under radio coverage of a mobile access network 300, which is divided into a radio network part 300a and a core network part 300b. 
The core network 300b shown in FIG. 3 includes a GGSN (Gateway GPRS Switching Node) 304 and a “policy unit” 306, often referred to as PDF (Policy Decision Function) or PCRF (Policy and Charging Rule Function). The policy unit is basically responsible for authorising communication sessions. Of course, network 300 contains numerous other nodes and elements that are not necessary to describe to understand the context of the present invention. For simplicity, the IMS network of terminal A is here merely represented as an “IMS core” 308, containing various nodes, not shown, involved in the procedures to be described below.
In a first stage 3:1, a basic PDP context, referred to as “primary”, is activated to obtain an IP connection. Activating the primary PDP context includes obtaining a RAB, for packet-switched SIP signalling messages over IP. The PDP context is created by GGSN 304. This RAB is typically based on so-called “best effort” communication with no particular requirements regarding bit rate and delay, since it is only intended to occasionally carry limited SIP messages.
In a next stage 3:2, terminal A registers with the IMS core 308, as basically handled by an S-CSCF node and HSS therein, not shown. The IMS registration involves a certain amount of SIP-based signalling over the primary PDP context.
Next, a multimedia session is to be established with party B in a following stage 3:3. In this stage, the above-mentioned protocol SDP is used within the SIP messages, such as INVITE, to communicate session-specific parameters including codec's, wherein some parameters may be indicated as preconditions.
Typically, a calling terminal proposes one or more codec's, along with other parameters, to use during the session, as specified in an INVITE message, and the called terminal responds by confirming a suitable proposed codec, and any other proposed parameters, in an “OK (invite)” message. Stage 3:3 further includes authorising the session in the policy unit 306, based on the session data and stored subscriber data. Stage 3:3 also includes a procedure for reserving communication resources in the mobile network 300 that are adapted to the forthcoming session with party B and according to parameters confirmed by both parties in their SIP dialogue.
The session establishment and resource reservation entail that a secondary PDP context is activated for terminal A, here indicated as a separate stage 3:4, which should be adapted for the media type(s) involved in the forthcoming session. The following QoS parameters may be indicated in the secondary PDP context: Traffic class, Maximum bit rate (uplink/downlink), Guaranteed bit rate (uplink/downlink), Transfer delay (uplink/downlink), Delivery order, Maximum SDU (Service Data Unit) size and a Source Statistic Descriptor.
The secondary PDP context is handled by GGSN in the same manner as for the primary PDP context in stage 3:1. Thus, the secondary PDP context should be defined so as to fulfil the requirements of the session with respect to the QoS parameter information as well as other factors, in order to obtain a proper RAB for media to be communicated. The new RAB is thus more stable and reliable as compared to the first one associated with the primary PDP context, and should provide a “guaranteed” QoS.
When the secondary PDP context has finally been established, the session must be acknowledged and the reserved resources be activated, as illustrated in a stage 3:5, before commencing the actual session in a final illustrated stage 3:6, over the secondary PDP context. Activating network resources is sometimes referred to as “opening of gates”.
The process of establishing a session, reserving network resources, activating the secondary PDP context and activating the reserved resources, as illustrated in stages 3:3-3:5, requires a significant amount of sequential signalling as dictated by standardised protocols. Moreover, a similar procedure must take place for the other party, at least if the other party is also a mobile terminal. In particular, stage 3:3 cannot be executed simultaneously at both sides, since the B-side in this case will reserve network resources before confirming session parameters to the B-side, according to prevailing standards. Thus, reserving network resources at the A-side must wait until confirmed session parameters have been received from the B-side.
The communication of media is thus delayed by the extensive sequential signalling required according to conventional set-up procedures for multimedia sessions. In the field of mobile communication, it is generally desirable to minimise such delays to make multimedia services more attractive to mobile end-users. For example, when using the service called “Push-to-talk over Cellular (PoC)”, which emulates a walkie-talkie service, users wish to talk immediately after pressing a push-to-talk button or similar, although this basically triggers the entire process of stages 3:3-3:5 above.
Further, the reservation of network resources is initiated by the mobile terminal and is therefore partly out of control for a network operator. It is thus generally desirable for network operators to gain full control of the allocation of network resources to different users.